Your shower goes hot, then cold,
then hot again

On a combi boiler, hot water at the tap is made as you draw it, through a component called the plate heat exchanger. A lot of the time, one of the causes of water cycling hot and cold is a blockage in there. The pattern you are feeling at the shower is the boiler overheating, shutting itself down, cooling, and firing again.

Why it cycles rather than just going cold

This is the part worth understanding, because it explains the rhythm.

The plate heat exchanger, which you may also see called the secondary heat exchanger or the domestic hot water heat exchanger, sits between the water the boiler heats and the water coming out of your tap. If it silts up, heat cannot move across it properly. The boiler keeps firing, the heat has nowhere to go, the boiler gets too hot and cuts out to protect itself. Then it cools down, starts again, and you get another slug of hot water before the same thing happens.

That is why it feels like a cycle. It is a cycle.

It is not always the boiler

Worth saying plainly, because we would rather you checked the free things first.

A combi boiler is generally only designed to serve one hot water tap at a time, so a shower that misbehaves while somebody is running water elsewhere in the house is usually doing what it was built to do.

We have also seen it where one faulty mixer outlet means none of the outlets get water that is hot enough, because that one faulty mixer is letting cold across into the hot and pushing cold water round to everything else. One tap can cause a problem you notice everywhere.

And if it is the plate

Where it does turn out to be a blocked plate to plate heat exchanger, that is a sign of a dirty system. We would advise getting a flush done, because otherwise this is likely to happen again.

That is not the answer anybody wants to hear. Cleaning or swapping the plate on a system full of debris buys you time and not much else, so we would rather tell you now than have you pay for the same job twice.

Not every boiler does it equally

Something we have noticed over the years, and it is the sort of thing you only see if you work on a lot of the same appliances in the same area.

Plate blockages are not spread evenly across manufacturers. We have changed and cleaned a good number of plates over the years, and on one of the brands we fit most often, in an area where it is a very popular boiler, I can think of exactly one blocked plate in the last ten years. I rate those plates very highly, and I have to give them credit for that. The brand is Vaillant. They still have other faults as commonly as anybody else, but they seem to have conquered the plate heat exchanger.

The other thing a damaged plate does

If the plate develops a pinhole instead of silting up, you get a different symptom entirely, and it is one people rarely connect to the hot water.

Mains cold water leaks across into the sealed heating side, and the pressure on your boiler gauge starts climbing on its own without anybody touching the filling loop. So if your pressure is going up by itself, and the usual suspect of a filling loop left slightly open has been ruled out, the plate is worth looking at.

What we check

The plate itself, whether it can be cleaned or wants replacing, the flow rate through it, and whether the rest of the system is dirty enough that it will silt up again.

When it is not a job to do yourself

Anything inside the appliance. The free checks are the ones above, about whether other taps do the same thing and whether it only happens when water is running elsewhere.

Quick answers